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 One of the main contributors is oxidization, i.e. too many oxygen atoms interacting with wine metals. Virginia Tech reports on a process called hyper-reduction that may offer newfound shelf life and spring to your wine's step. GIVE YOUR WINE NEW LIFE |
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CONTROLLING OXIDATION IN WINE Reproduced with permission from the Department of Food Science and Technology, Virginia Tech Author: Bruce Zoecklein, Associate Professor and Enology Specialist, Department of Food Science and Technology, Virginia Tech Why do some wines seem to fade much more quickly than expected? There can be a number of reasons for this; many have been outlined in previous editions of Enology Notes. Dr. Andy Waterhouse has aided our understanding of wine oxidation and the role of metals. The following is a brief review. When wine compounds combine with oxygen, they can pick up one or more oxygen atoms and become “oxidized.” These new compounds are now different, and can have different sensory characteristics. For example, ethanol can be oxidized to acetaldehyde and further, to acetic acid, each with very different sensory features. Similarly, polyphenols can be oxidized to quinones, and metals such as copper, iron, and manganese can be transformed from Cu+ to Cu2+, Fe2+ to Fe3+, and Mn2+ to Mn3+, respectively, with potential sensory changes. These same multivalent metals can also act as catalysts in oxidation reactions. Oxygen can complex with these metals, and they have been identified in intermediate oxidation products. The traditional view of wine oxidation is coupled oxidation, where phenols are oxidized to form quinones (brown phenols) and produce hydrogen peroxide. The hydrogen peroxide goes on to react with a dominant species in wine, such as ethanol, to form acetaldehyde as depicted below.  Iron and copper are both strong oxidizers. The potential oxidizing effect is illustrated by the Fenton-type reaction: The OH*, or free hydroxyl radical is formed in the presence of iron and likely other metals. It is the most oxidative species that can react with a number of wine components, impacting the sensory features and longevity. An important implication of the presence of hydroxyl radical is the formation of numerous aldehydes and ketones from oxidation of alcohols. These alcohol derivatives react with flavonoid phenols, creating linkages that may help stabilize color and create bonds between tannins, and between tannins, proteins, and polysaccharides. The control and extent of oxidative reactions is the important issue. It is likely that a relatively high concentration of metals inhibited practices such as microoxygenation in the past. When I first entered the industry it was not an un- common practice to add citric acid to all wines to help bind or chelate iron to help prevent iron casse formation. High concentrations of iron, for example, can render a wine “over the hill” following only limited oxygen exposure. The formation of the reactive hydroxyl radical (OH*) requires a metal catalyst such as Fe+2. The Fe+3 produced in the above reaction is converted back to Fe+2 to continue the cycle. This conversion is caused by certain phenols. As such, phenols are acting to accelerate the oxidation process. Additionally, antioxidants do not have any controlling impact on the rate of this reaction. It is important that winemakers understand the sources of metal pick-up (equipment, fining agents, additives, water) and the possible role that metals, such as iron, can have on their wines. SHARE THIS ARTICLE ... 
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